Antennas with variable electrical tilt (VET) functionality are known in the art. These antennas, which often are used in cellular networks, enable network operators to tilt the elevation beam pointing direction of an antenna by manually rotating a knob or translating a shaft on the exterior of the antenna. The knob or shaft is linked to phase shifters inside the antenna to convert the mechanical rotation or translation of the shaft to phase changes in the radio frequency beam forming network inside the antenna. Changes in phase between radiating elements inside the antenna cause the beam emitted from the antenna to tilt up or down relative to mechanical boresite of the antenna. An example of a cellular base station antenna demonstrating VET technology is depicted in U.S. Pat. No. 7,068,236, which is incorporated by reference.
Beam tilt adjustment is needed in cellular networks to reduce signal propagation between sites in the network in order to minimize signal interference and to maximize network capacity. Antennas with VET functionality allow network operators to make accurate tilt adjustments at a cell site without mechanically tilting the antenna and without changing the visual appearance of the site. Antennas with VET functionality typically include some sort of tilt indicator to provide visual feedback of the antenna electrical tilt setting to the person making the tilt adjustment.
Remote electrical tilt (RET) antennas are also known in the art. RET antennas incorporate an electro-mechanical actuator attached to or installed inside of the antenna to rotate the knob or translate the shaft on a VET antenna. This enables the electrical tilt of the VET antenna to be controlled from a remote location, eliminating the expense of hiring a rigger to climb the tower and manually adjust the electrical tilt of the antenna beam.
With most RET antennas, the electro-mechanical actuator attaches to the VET antenna at or near the manual tilt adjustment mechanism of the antenna. With the RET actuator installed, the tilt of the antenna can no longer be adjusted manually. In order to manually adjust the tilt of the antenna, the RET actuator must be physically removed or separated from the antenna to provide access to the manual tilt adjustment mechanism. Removing the actuator is often tedious and time consuming due to small attachment screws and delicate interface components. In addition, calibration can be lost between the electro-mechanical actuator and the antenna tilt setting once the actuator is removed. A calibration sequence must to be run to re-calibrate the actuator to the antenna before proper remote operation can be restored.
An improvement on the standard RET antenna design is depicted in U.S. Pat. No. 7,286,092, which is incorporated by reference. In this design, the electro-mechanical actuator inserts inside the antenna body without blocking access to the manual tilt adjustment mechanism of the antenna. With the RET actuator engaged, the tilt of this antenna can be adjusted manually using a 10 mm wrench to rotate the tilt adjustment mechanism. In this design, the wrench is needed to overcome the mechanical resistance of the phase shifters plus the mechanical resistance of the motor. In this design, the motor does not separate from the drive chain during a manual tilt operation.
A low gear ratio drive unit is required in this design to enable manual tilt adjustment with the RET actuator is installed. The low gear ratio drive unit allows the motor and the drive unit to be manually turned with relatively low drive torque. If a high gear ratio drive unit were used, it would be very difficult to adjust the tilt manually and the high torque required to manually back-drive the motor and the drive unit could potentially break the plastic teeth on the tilt indicator drive shaft.
A problem with using low gear ratio drive units, however, is that a higher torque DC motor is required to generate sufficient torque to operate the RET actuator during remote tilt operation. The high torque DC motor is expensive by itself. In addition, the motor draws high current during tilt operations forcing the use of expensive, high current rated components on the controller circuit.
There is, therefore, a continuing need for a RET antenna that uses low current, low torque DC motors to reduce cost without losing the ability to manually tilt the antenna when the electro-mechanical actuator is installed. A further need exists for a RET antenna that allows for both electro-mechanical and manually tilt adjustment without losing the tilt calibration of the antenna.